Latitudinal gradient, MEND experiment, and BioGen experiment relating species richness and net primary productivity (NPP)

Experiments often find that net primary productivity (NPP) increases with
species richness when native species are considered.  However,
relationships may be altered by exotic (non-nattive) species, which are
hypothesized to reduce richness but increase productivity
(invasion-diversity-productivity' paradox).  We compared
richness-NPP relationships using a comparison of exotic vs.
native-dominated sites across the central USA, and two epxeriments under
common enviornments.  ANPP was measured using peak biomasss
clipping in all three studies.  In all studies, there was a
signfiicantly positive relationship between NPP and richness across
native-species doimnated sites and plots, but no relatoinship across
exotic-dominated ones.  The results indicatte that relationships
between NPP and richness depend on whether natieve or exotic species are
dominant, and that exotic species are 'breaking the rules'.